export rfc2109_to_date/1
add date tests (90% code coverage)
	modified:   src/hackney_date.erl
	new file:   test/hackney_date_tests.erl
...
	geändert:   src/hackney_headers.erl
	neue Datei:   test/hackney_headers_tests.erl
some more _headers tests
	geändert:   test/hackney_headers_tests.erl
Merge pull request #10 from wactbprot/master

add unit tests for date module