fix namespace prefix deserialize

[KaiserY]

Similar to #539, namespace prefix (e.g. r:id) also map to @r:id instead of @id, possibly fix #591.

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@@            Coverage Diff             @@
##           master     #797      +/-   ##
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- Coverage   61.81%   60.10%   -1.72%     
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  Files          41       41              
  Lines       16798    16100     -698     
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- Hits        10384     9677     -707     
- Misses       6414     6423       +9     

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[Mingun]

The problem with that approach that in the real document prefix not necessary will be called r. This is different from the xmlns prefix, which is a special prefix that is always defined, cannot be redefined and its namespace cannot be assigned to another prefix. In other words, all of that are prohibited:

• xmlns:xmlns="something-not-xmlns-namespace"
• xmlns:xmlns="http://www.w3.org/2000/xmlns/"
• xmlns:any-name="http://www.w3.org/2000/xmlns/"

In the end that means that the only way to declare namespace prefix is to use xmlns:prefix-name="namespace". xmlns is constant. That is why we have an exception for it, although you generally should not read namespace prefixes into the fields, because prefix-name can be arbitrary XML identifier and rely on the particular name for it is dangerous.

The right approach is to include namespace name to the #[serde(rename)] and during deserialization resolve actual prefixes into namespaces, compose a field name from them (for example, @attr{namespace}) and pass that name to the type's Visitor. That already was explored in #466 and we should move forward that way.

[KaiserY]

Thanks for the full explanation. in my case i'm parsing ooxml, somehow the prefix seem fixed, so i think this will be a quick fix. I will dig more into #466, close this for now.