Use struct instead of class tag for explicit template instantiation

[patlkli]

After trying to compile fastcgipp with a more recent version of clang (6.0 in this case) I encountered several warnings, which resulted in fatal errors thanks to -Werror. This pull request fixes #35 where those warnings were already reported.

[...]/fastcgipp/src/http.cpp:626:10: error: class template 'Environment' was previously declared as a
      struct template [-Werror,-Wmismatched-tags]
template class Fastcgipp::Http::Environment<char>;
         ^
[...]/fastcgipp/include/fastcgi++/http.hpp:251:38: note: previous use is here
        template<class charT> struct Environment
                                     ^
[...]/fastcgipp/src/http.cpp:626:10: note: did you mean struct here?
template class Fastcgipp::Http::Environment<char>;
         ^~~~~
         struct
[...]/fastcgipp/src/http.cpp:627:10: error: class template 'Environment' was previously declared as a
      struct template [-Werror,-Wmismatched-tags]
template class Fastcgipp::Http::Environment<wchar_t>;
         ^
[...]/fastcgipp/include/fastcgi++/http.hpp:251:38: note: previous use is here
        template<class charT> struct Environment
                                     ^
[...]/fastcgipp/src/http.cpp:627:10: note: did you mean struct here?
template class Fastcgipp::Http::Environment<wchar_t>;
         ^~~~~
         struct

[eddic]

Thanks!