Parse the MIME media type from the Content-Type header

[JayH5]

• Rather than just checking the raw header value
• Don't parse the header if we're streaming everything anyway

[JayH5]

It's quite common for SSE sources to also specify the character set in the Content-Type header (even though the SSE spec only allows one character set). i.e. Content-Type: text/event-stream; charset=utf-8.

It's also technically correct to pass a Content-Type header value with leading or trailing whitespace or with mixed character case.

This PR uses mime.ParseMediaType to parse the MIME type from the Content-Type header correctly.

[emilevauge]

Ouch, how did I miss that 😢 ...
Thanks for catching this @JayH5 !
LGTM