cli/command: don't use client.CustomHTTPHeaders()

[thaJeztah]

It's the only use of this function, and it's better to check that
the client actually sends the header.

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Hmm... looks like this fails currently, because golang only resolves the proxy configuration once (using a sync.Once) and because this test actually makes a request, expectations for other tests fail 😞

--- FAIL: TestNewAPIClientFromFlagsWithHttpProxyEnv (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x67898a]

goroutine 49 [running]:
testing.tRunner.func1(0xc000334a00)
	/usr/local/go/src/testing/testing.go:874 +0x3a3
panic(0xb708a0, 0x12844d0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
net/url.(*URL).String(0x0, 0xc000334a00, 0x0)
	/usr/local/go/src/net/url/url.go:775 +0x4a
github.com/docker/cli/cli/command.TestNewAPIClientFromFlagsWithHttpProxyEnv(0xc000334a00)
	/go/src/github.com/docker/cli/cli/command/cli_test.go:111 +0x39c
testing.tRunner(0xc000334a00, 0xc94ba0)
	/usr/local/go/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:960 +0x350
FAIL	github.com/docker/cli/cli/command	0.013s

[codecov-commenter]

Codecov Report

Merging #3203 (cc709c3) into master (c9f8473) will not change coverage.
The diff coverage is n/a.

❗ Current head cc709c3 differs from pull request most recent head 0b2eaa7. Consider uploading reports for the commit 0b2eaa7 to get more accurate results

@@           Coverage Diff           @@
##           master    #3203   +/-   ##
=======================================
  Coverage   58.58%   58.58%           
=======================================
  Files         299      299           
  Lines       21502    21502           
=======================================
  Hits        12597    12597           
  Misses       7983     7983           
  Partials      922      922           

[silvin-lubecki]

LGTM 👍