Prefer 'close' over 'keep-alive' when both are present

[zarqman]

If the Connection header has both close and keep-alive, both related methods return true despite the conflicting instructions.

This PR changes keep_alive? to return false if close is simultaneously present.

Related to socketry/async-http#132

Types of Changes

• Bug fix.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

What an interesting behaviour. Apparently Chrome and Firefox interpret this differently. https://stackoverflow.com/questions/11443669/how-to-interpret-connection-keep-alive-close