🐛 fix gherkin lexical_cast for string input parameters

[ambushed]

Problem:

• when a string parameter is parsed in a gherkin test clause, ut is ignoring
  everything after the first whitespace character

Solution:

• introduce a special case for std::string in the lexical_cast
  method of gherkin test not to rely on the streaming input operator
  (works only until the first whitespace) but to take the whole value.

[kris-jusiak]

thanks @ambushed