net/http/httputil: handle escaped paths in SingleHostReverseProxy

[dkumor]

When forwarding a request, a SingleHostReverseProxy appends the
request's path to the target URL's path. However, if certain path
elements are encoded, (such as %2F for slash in either the request or
target path), simply joining the URL.Path elements is not sufficient,
since the field holds the decoded path.

Since 87a605, the RawPath field was added which holds a decoding
hint for the URL. When joining URL paths, this decoding hint needs
to be taken into consideration.

As an example, if the target URL.Path is /a/b, and URL.RawPath
is /a%2Fb, joining the path with /c should result in /a/b/c
in URL.Path, and /a%2Fb/c in RawPath.

The added joinURLPath function combines the two URL's Paths,
while taking into account escaping, and replaces the previously used
singleJoiningSlash in NewSingleHostReverseProxy.

Fixes #35908

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[dkumor]

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Patch Set 1:

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Message from Brad Fitzpatrick:

Patch Set 1: Code-Review+1

(2 comments)

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[dkumor]

Hi, just wanted to check on this PR, and make sure it isn't lost in the current chaos. I have implemented the requested modifications - please let me know if there is anything more I can do to help in fixing the issue!

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Message from Daniel Kumor:

Patch Set 2:

(2 comments)

Hi, just wanted to check on this PR, and make sure it isn't lost in the current chaos. I have implemented the requested modifications in patch set 2 - please let me know if there is anything more I can do to help in fixing the issue!

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Message from Ian Lance Taylor:

Patch Set 2: Run-TryBot+1

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Patch Set 2:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=686781f0

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Patch Set 2: TryBot-Result+1

TryBots are happy.

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Message from Brad Fitzpatrick:

Patch Set 2: Code-Review+2

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This PR is being closed because golang.org/cl/213257 has been merged.