[swift5] fix use of isBasic condition

[tiffmaelite]

Follow-up of #15220 but for Swift

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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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

Hi, thanks for taking the time to contribute to openapi-generator.
Looks good to me 👍