[dart] fix use of isBasic condition

[tiffmaelite]

Follow-up of #15220 but for dart

isBasic is also true for HTTP bearer auth method and HTTP signature method, not only HTTP basic auth method; it should not be used when we want to apply code changes in the context of HTTP basic auth only!

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[ahmednfwela]

@tiffmaelite what is the difference between isBasicBasic and isBasic?

[tiffmaelite]

@tiffmaelite what is the difference between isBasicBasic and isBasic?

isBasic is true for HTTP bearer auth method and HTTP signature method, not only HTTP basic auth method
I edited the PR description