[mockserver] Allow to set the content-type of String responses

[martinbonnin]

A bunch of mockserver fixes:

• Allow to set the content-type of String responses
• Fix reading content-length (it should be case insensitive)
• More robust (albeit prone to false positive) handling of connection close

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[BoD]

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[martinbonnin]

I reverted the change to store headers lowercase as it could be surprising. As always, okhttp has a good API for headers but I decided to not go all-in because it's a breaking change and we're not really a HTTP library.

If we decide to make mockserver first class citizen, we should move it out of the repo and make the API better. Probably also share HttpEngine with the runtime.

[BoD]

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