Improve error message on invalid headers

[wojtekmach]

Ref: elixir-mint/mint#468

With this patch in Mint we now have:

iex> {:ok, conn} = Mint.HTTP2.connect(:https, "httpbin.org", 443) ; Mint.HTTP.request(conn, "GET", "/json", [{"accept", nil}], nil)
** (ArgumentError) expected header name/value to be strings, got: "accept"/nil
    (hpax 1.0.3) lib/hpax.ex:339: HPAX.validate_header/2
    (hpax 1.0.3) lib/hpax.ex:302: HPAX.encode_headers/3
    (elixir 1.18.4) lib/map.ex:999: Map.get_and_update!/3
    (mint 1.7.1) lib/mint/http2.ex:1134: Mint.HTTP2.encode_headers/4
    (mint 1.7.1) lib/mint/http2.ex:520: Mint.HTTP2.request/5
    iex:1: (file)

[coveralls]

Pull Request Test Coverage Report for Build 138451c6e3952dc77e541617d229a7e614bc0940-PR-24

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 95.808%

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Totals
Change from base Build bbc9fe6d60a4a5313270792bdd02ee12536348f9:	0.08%
Covered Lines:	160
Relevant Lines:	167

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💛 - Coveralls

[whatyouhide]

Can we add a test for this @wojtekmach? Looks great otherwise

[whatyouhide]

Thanks @wojtekmach 💟