Revert "JSON string output funtions should clear() the destination first"

We found that some code depended on the behavior of appending to the
string without clearing it first. The comments in json_util.h also make
it appear that the appending behavior is intentional, so I think we need
to stick with the old behavior.

This reverts commit 3cbe45a4b5.

src/google/protobuf/util/json_util.cc

@@ -115,7 +115,6 @@ util::Status BinaryToJsonString(TypeResolver* resolver,
                                   const std::string& binary_input,
                                   std::string* json_output,
                                   const JsonPrintOptions& options) {
-  json_output->clear();
   io::ArrayInputStream input_stream(binary_input.data(), binary_input.size());
   io::StringOutputStream output_stream(json_output);
   return BinaryToJsonStream(resolver, type_url, &input_stream, &output_stream,
@@ -210,7 +209,6 @@ util::Status JsonToBinaryString(TypeResolver* resolver,
                                   StringPiece json_input,
                                   std::string* binary_output,
                                   const JsonParseOptions& options) {
-  binary_output->clear();
   io::ArrayInputStream input_stream(json_input.data(), json_input.size());
   io::StringOutputStream output_stream(binary_output);
   return JsonToBinaryStream(resolver, type_url, &input_stream, &output_stream,