Allow specifying header on a per-request basis

[AdamGS]

Is your feature request related to a problem? Please describe.
I want to be able to specify headers on a per request headers, in my specific use cases those are if-none-match and if-match as different object require different handling but I can imagine it being useful for other usescases (content-type being an obvious one).

Describe the solution you'd like
have _with_headers variants for some functions that allow for specifying a second header override (in addition to the bucket level additional_headers

Describe alternatives you've considered
Another possibility is storing some closure/middleware in Bucket that will check requests on their way out and mutate them as required.