SPR-15066 - Allow request interceptor to add to headers set via entity

[einarpehrson]

Provide a fully mutable HttpHeaders to ClientHttpRequestInterceptors of a RestTemplate when headers are set using HttpEntity. This avoids UnsupportedOperationException if both HttpEntity and ClientHttpRequestInterceptor add values for the same HTTP header.

I came upon this error while investigating a bug report in a third-party starter I maintain. The user was using a RestTemplate where our starter (without the user's knowledge) provided a ClientHttpRequestInterceptor that added an Authorization header. The user rather wanted to use a different authentication method and provided an Authorization header with a different scheme through an HttpEntity passed to the RestTemplate. I am avoiding this problem in our starter by calling HttpHeaders#set(String, String) instead of #add(String, String).

But seeing that HttpEntityRequestCallback was copying over headers from the provided HttpEntity to another HttpHeaders object used for the request, I thought the intended behavior was rather to provide a fully mutable HttpHeaders for the remainder of the processing of the request, including any invocation of ClientHttpRequestInterceptors.

On the other hand, this is a marginal use case at best, and could even be considered misuse of the API.

Issue: SPR-15066

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[einarpehrson]

Declining this PR since @sdeleuze pushed these changes in his commits 554662e and 73a81f9.