Properly close 'file://' resources

[CharString]

I bumped into this when opening several wsdl which in turn opened lots
of xsd, with 'file://' scheme.

The issue was that the resp.raw.close nor resp.raw.release_conn set
in the FileAdapter were ever called.

It's unclear to me whether this should be fixed in requests. It doesn't
do that great a job at resource management for the naive user aka
Human™. It makes sense to me that exhaustively reading Response.raw
should close it unless the caller explicitly set stream on the
request. Probably by using this closing pattern in the generator in
Response.iter_content.

Workarounds without this fix:

• using a scheme-less url as zeep will assume it's a local path and
  open the file as a context manager.
• use one of the caches from zeep.cache to hide duplicate open resources.