Fix #138: validate HTTP headers.

[mmaker]

No description provided.

[coveralls]

Coverage decreased (-0.02%) when pulling 187b4b7 on mmaker:master into 5582dd3 on fiorix:master.

[fiorix]

Why have this line:

content_length = int(headers.get("Content-Length", 0))

in the try/except block?

[mmaker]

Because it fixes the following error shown in #138:
exceptions.ValueError: invalid literal for int() with base 10: '\xc3\xbc10'

[fiorix]

Thanks