fix(fetch/axios): correctly merge headers from opts

[mnahkies]

• Previously if you passed any request headers using the opts parameter, this would overwrite the headers set from parameters, defaults, etc
• Now these will be consistently merged following: defaults -> params -> opts
  • undefined header values will be ignored, null header values will delete headers set previously

This wound up being a lot more complex than I'd expected, and there may still be some edges that I've missed. However I'm confident that this makes things better than before based on the new e2e tests at least.