Pass library and Python version in x-goog-api-client

[busunkim96]

Closes #729

[asiunov]

@busunkim96 @tseaver @dazuma
seems like this PR introduces some regression. I started to get a signature mismatch when use this sdk (on .get_media method):

W0813 19:40:45.663586 1 http.py:118] Invalid JSON content from response: b"<?xml version='1.0' encoding='UTF-8'?><Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match the signature you provided. Check your Google secret key and signing method.</Message><StringToSign>GET\n\n\n1565725545\nx-goog-api-client:gdcl/1.7.11 gl-python/3.7.4\n/...</StringToSign></Error>"

[busunkim96]

@asiunov Thank you for the report! Could you let us know which API you are using? (A small sample would also be much appreciated).

I see a similar error mentioned for node, but am unsure if this is the same cause.