Miscellaneous patches

Note that I haven't run the code - this is all straight from dialyzer's mouth.  But hey, some of these _are_ problems...

*hackney.erl*
1. correct spec to allow URL to be url() | binary()
   * just making this explicit
1. Possible typo in header generation
   * in ```connect_proxy```, you probably meant to generate a _new_ value for Host.  The code, as it exists, probably would have barfed.
1. The return from skip_body is actually {ok, _}| {error _}
   * ```skip_body``` returns {ok, #client{}} | {error, any()}.  The code, as it exists, wouldn't have worked.

*hackney_pool.erl*
1.dict:find returns error, not false
   * I _know_ that needs to be fixed :-)

*hackney_url.erl*
1. Added 'nil' handling
   * _Theoretically_, the code path allows a 'nil' value for User to make it through to this point