Pass string as a const reference.

Author: Nekto89

include/json/writer.h

@@ -300,7 +300,7 @@ class JSONCPP_DEPRECATED("Use StreamWriterBuilder instead") JSON_API
   /**
    * \param indentation Each level will be indented by this amount extra.
    */
-  StyledStreamWriter(JSONCPP_STRING indentation = "\t");
+  StyledStreamWriter(const JSONCPP_STRING& indentation = "\t");
   ~StyledStreamWriter() {}
 
 public:

src/lib_json/json_writer.cpp

@@ -643,7 +643,7 @@ bool StyledWriter::hasCommentForValue(const Value& value) {
 // Class StyledStreamWriter
 // //////////////////////////////////////////////////////////////////
 
-StyledStreamWriter::StyledStreamWriter(JSONCPP_STRING indentation)
+StyledStreamWriter::StyledStreamWriter(const JSONCPP_STRING& indentation)
     : document_(NULL), rightMargin_(74), indentation_(indentation),
       addChildValues_(), indented_(false) {}