fix(storage): use regex in v4SanitizeHeaders

[rotemlinik]

The v4SanitizeHeaders() logic used split(":") func to separate the headers key:val, but in cases where the value itself contains : (such as dates in RFC3339 format as required), the value is partly lost.
I replaced the logic with a regex logic- same as the one used in v2SanitizeHeaders() and added a unit test for this case.

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[tritone]

I think not using the canonicalHeaderRegexp is actually an intentional difference between v2 and v4; see comments in L526-527.

However, maybe we should use strings.SplitN instead so that only the first : is split on. Does that sound like a good solution to you?

[BrennaEpp]

Closing this as #7603 contains the intended changes.

As of release v1.30.1, storage allows the use of values containing : in v4 headers