Invalid header CRLF

When a request comes in that doesn't use the correct \r\n between headers in HTTP we get the following error:
```
hermes.104453: Error in process <0.14992.7504> on node 'hermes@ip-10-120-198-130.ec2.internal' with exit value: {{case_clause,<<309 bytes>>},[{cowboy_protocol,parse_hd_value,9,[{file,"src/cowboy_protocol.erl"},{line,405}]}]}
```
A header that causes this could is something like this:
```
HeaderName: HeaderValue\rHeaderName2: HeaderValue
```
Notice the lack of CRLF.

This commit causes this to return a 400 error.