mime/multipart: add field Reader.MaxMIMEHeaderSize

[starius]

If the field is set, it is used instead of maxMIMEHeaderSize
constant, allowing to further constraint memory usage when
parsing multipart streams.

Fixes #68889
Fixes #73087
Fixes #26339

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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[sdavids]

This constant should be mentioned in the doc of ParseMultipartForm.

[starius]

This constant should be mentioned in the doc of ParseMultipartForm.

I updated the PR. Added notes to methods request.MultipartReader and request.ParseMultipartForm.

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[gopherbot]

Message from Борис Нагаев:

Patch Set 2:

(1 comment)

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