[#] Only copy the content encoding and type, not all headers to fix issues with chunked responses. Improve overall performance and error reporting.
[#896] Make json preview ajax request async to prevent blocked ui
[#896] Show the original error from the response instead of limited message.
[#896] Require latest httpretty in tests to fix proxy tests
[#896] Do a GET request if HEAD is not supported
Merge branch 'master' into 896-resource-proxy-fixes
[#896] Fix failing test, fix PEP8 issues
Merge branch '896-resource-proxy-fixes'

Conflicts:
	ckanext/jsonpreview/theme/public/preview_json.js