feat: more strictly coerce strings, exclude arrays and plain objects

BREAKING CHANGE: plain objects and arrays are no long cast to strings automatically

to recreate the old behavior:
```js
string().transform((_, input) => input != null && input.toString ? input.toString() : value);
```

src/string.js

@@ -8,10 +8,12 @@ let rEmail = /^((([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\u
 // eslint-disable-next-line
 let rUrl = /^((https?|ftp):)?\/\/(((([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(%[\da-f]{2})|[!\$&'\(\)\*\+,;=]|:)*@)?(((\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\.(\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\.(\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\.(\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5]))|((([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.)+(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.?)(:\d*)?)(\/((([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(%[\da-f]{2})|[!\$&'\(\)\*\+,;=]|:|@)+(\/(([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(%[\da-f]{2})|[!\$&'\(\)\*\+,;=]|:|@)*)*)?)?(\?((([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(%[\da-f]{2})|[!\$&'\(\)\*\+,;=]|:|@)|[\uE000-\uF8FF]|\/|\?)*)?(\#((([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(%[\da-f]{2})|[!\$&'\(\)\*\+,;=]|:|@)|\/|\?)*)?$/i;
 // eslint-disable-next-line
-let rUUID = /^(?:[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}|00000000-0000-0000-0000-000000000000)$/i
+let rUUID = /^(?:[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}|00000000-0000-0000-0000-000000000000)$/i;
 
 let isTrimmed = (value) => isAbsent(value) || value === value.trim();
 
+let objStringTag = {}.toString();
+
 export default function StringSchema() {
   if (!(this instanceof StringSchema)) return new StringSchema();
 
@@ -20,7 +22,14 @@ export default function StringSchema() {
   this.withMutation(() => {
     this.transform(function (value) {
       if (this.isType(value)) return value;
-      return value != null && value.toString ? value.toString() : value;
+      if (Array.isArray(value)) return value;
+
+      const strValue =
+        value != null && value.toString ? value.toString() : value;
+
+      if (strValue === objStringTag) return value;
+
+      return strValue;
     });
   });
 }
@@ -33,9 +42,7 @@ inherits(StringSchema, MixedSchema, {
   },
 
   _isPresent(value) {
-    return (
-      MixedSchema.prototype._isPresent.call(this, value) && value.length > 0
-    );
+    return MixedSchema.prototype._isPresent.call(this, value) && !!value.length;
   },
 
   length(length, message = locale.length) {

test/string.js

@@ -10,14 +10,20 @@ describe('String types', () => {
       valid: [
         [5, '5'],
         ['3', '3'],
-        //[new String('foo'), 'foo'],
+        // [new String('foo'), 'foo'],
         ['', ''],
         [true, 'true'],
         [false, 'false'],
         [0, '0'],
         [null, null, schema.nullable()],
+        [
+          {
+            toString: () => 'hey',
+          },
+          'hey',
+        ],
       ],
-      invalid: [null],
+      invalid: [null, {}, []],
     });
 
     describe('ensure', () => {