Implement missing string conversions for JSON

`Json::Value::isConvertibleTo` indicates that unsigned integers and
reals are convertible to string, but trying to do so (with
`Json::Value::asString`) throws an exception because its internal switch
is missing these cases. This change fills them in (and adds tests).

Acknowledgements:
Ripple thanks Guido Vranken for responsibly disclosing this issue.

Closes #2778

src/ripple/json/impl/json_value.cpp

@@ -523,7 +523,11 @@ Value::asString () const
         return beast::lexicalCastThrow <std::string> (value_.int_);
 
     case uintValue:
+        return beast::lexicalCastThrow <std::string> (value_.uint_);
+
     case realValue:
+        return beast::lexicalCastThrow <std::string> (value_.real_);
+
     case arrayValue:
     case objectValue:
         JSON_ASSERT_MESSAGE ( false, "Type is not convertible to string" );

src/ripple/json/json_value.h

@@ -246,12 +246,14 @@ class Value
     ValueType type () const;
 
     const char* asCString () const;
+    /** Returns the unquoted string value. */
     std::string asString () const;
     Int asInt () const;
     UInt asUInt () const;
     double asDouble () const;
     bool asBool () const;
 
+    // TODO: What is the "empty()" method this docstring mentions?
     /** isNull() tests to see if this field is null.  Don't use this method to
         test for emptiness: use empty(). */
     bool isNull () const;

src/test/json/json_value_test.cpp

@@ -24,6 +24,7 @@
 #include <ripple/beast/type_name.h>
 
 #include <algorithm>
+#include <regex>
 
 namespace ripple {
 
@@ -250,6 +251,41 @@ struct json_value_test : beast::unit_test::suite
         }
     }
 
+    void test_conversions ()
+    {
+      // We have Json::Int, but not Json::Double or Json::Real.
+      // We have Json::Int, Json::Value::Int, and Json::ValueType::intValue.
+      // We have Json::ValueType::realValue but Json::Value::asDouble.
+      // TODO: What's the thinking here?
+      {
+        // null
+        Json::Value val;
+        BEAST_EXPECT(val.isNull());
+        BEAST_EXPECT(val.asString() == "");
+      }
+      {
+        // bool
+        Json::Value val = true;
+        BEAST_EXPECT(val.asString() == "true");
+      }
+      {
+        // int
+        Json::Value val = -1234;
+        BEAST_EXPECT(val.asString() == "-1234");
+      }
+      {
+        // uint
+        Json::Value val = 1234U;
+        BEAST_EXPECT(val.asString() == "1234");
+      }
+      {
+        // real
+        Json::Value val = 2.0;
+        BEAST_EXPECT(std::regex_match(
+              val.asString(), std::regex("^2\\.0*$")));
+      }
+    }
+
     void test_nest_limits ()
     {
         Json::Reader r;
@@ -316,6 +352,7 @@ struct json_value_test : beast::unit_test::suite
         test_move ();
         test_comparisons ();
         test_compact ();
+        test_conversions();
         test_nest_limits ();
         test_leak();
     }