http: code cleanup & dry

The first change in `_writeRaw`:
This reduces drops an unnecessary if check (`outputLength`),
because it is redone in `_flushOutput`. It also changes an if/else
statement to an if statement, because the blocks were unrelated.

The second change in `write`:
This consolidates code in #write() that handled different
string encodings and Buffers. There was no reason to handle the
encodings differently, so after splitting them based on Buffer vs
encoding, the code is consolidated. This might see a speedup. Shoutout
to Ron Korving <ron@ronkorving.nl> for spotting this.

lib/_http_outgoing.js

@@ -155,10 +155,10 @@ function _writeRaw(data, encoding, callback) {
       connection.writable &&
       !connection.destroyed) {
     // There might be pending data in the this.output buffer.
-    var outputLength = this.output.length;
-    if (outputLength > 0) {
-      this._flushOutput(connection);
-    } else if (data.length === 0) {
+    this._flushOutput(connection);
+
+    // Avoid writing empty messages, but trigger the callback.
+    if (data.length === 0) {
       if (typeof callback === 'function')
         process.nextTick(callback);
       return true;
@@ -472,25 +472,16 @@ OutgoingMessage.prototype.write = function write(chunk, encoding, callback) {
 
   var len, ret;
   if (this.chunkedEncoding) {
-    if (typeof chunk === 'string' &&
-        encoding !== 'hex' &&
-        encoding !== 'base64' &&
-        encoding !== 'latin1') {
+    if (typeof chunk === 'string') {
       len = Buffer.byteLength(chunk, encoding);
-      chunk = len.toString(16) + CRLF + chunk + CRLF;
-      ret = this._send(chunk, encoding, callback);
     } else {
-      // buffer, or a non-toString-friendly encoding
-      if (typeof chunk === 'string')
-        len = Buffer.byteLength(chunk, encoding);
-      else
-        len = chunk.length;
-
-      this._send(len.toString(16), 'latin1', null);
-      this._send(crlf_buf, null, null);
-      this._send(chunk, encoding, null);
-      ret = this._send(crlf_buf, null, callback);
+      len = chunk.length;
     }
+
+    this._send(len.toString(16), 'latin1', null);
+    this._send(crlf_buf, null, null);
+    this._send(chunk, encoding, null);
+    ret = this._send(crlf_buf, null, callback);
   } else {
     ret = this._send(chunk, encoding, callback);
   }