[http] Use +"" for a mutable string

[juharris]

To avoid warnings I get in my console:

/Users/jus/.gem/ruby/3.4.1/gems/anthropic-0.3.2/lib/anthropic/http.rb:104: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
/Users/jus/.gem/ruby/3.4.1/gems/anthropic-0.3.2/lib/anthropic/http.rb:104: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
/Users/jus/.gem/ruby/3.4.1/gems/anthropic-0.3.2/lib/anthropic/http.rb:104: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

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