Fixed replace(inPlace) function

and added tests (should have done that in the first place - I'll never learn!)

include/internal/catch_common.h

@@ -71,6 +71,7 @@ namespace Catch {
     void toLowerInPlace( std::string& s );
     std::string toLower( std::string const& s );
     std::string trim( std::string const& str );
+    bool replaceInPlace( std::string& str, std::string const& replaceThis, std::string const& withThis );
 
     struct pluralise {
         pluralise( std::size_t count, std::string const& label );