Refactoring of headers breaks backwards compatibility

[Carsten-Leue]

Describe the bug

In this commit 7a2f901#diff-243f86a2e64131743ca04681df5d21438e79fcabe06f18029cb8dcef6a70121fR351

the type of AxiosRequestConfig got changed such that it is not longer possible to used indexed access to the headers.

error TS2551: Property 'Content-Type' does not exist on type 'AxiosHeaders | AxiosResponseHeaders | Partial<RawAxiosHeaders & MethodsHeaders & CommonHeaders> | Partial<...>'. Did you mean 'setContentType'?

const exHeader = headers['content-type'] || headers[KEY_CONTENT_TYPE];

This affects code that tried to read headers from the config

To Reproduce

config.headers['content-type']

where config is of type AxiosRequestConfig

Code snippet

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Expected behavior

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Axios Version

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Adapter Version

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Browser

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Node.js Version

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OS

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Additional context/Screenshots

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[bobvandevijver]

Probably related to #5416.