change GetResource to return the full resource metadata w/ bytes

[natefinch]

We changed to use a multipart body to return the data, so we don't have to use a bunch of ugly headers.

Also did a little cleanup of some spots where we were unnecessarily reading the full body into memory.

[jujugui]

Test PASSed.
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Test PASSed.

[natefinch]

In an external discussion, we have decided to drop the multi-part body approach and instead simply do two requests against the charmstore, one for the bytes and one for the metadata.