Fix #49169: SoapServer calls wrong function, although "SOAP action" header is correct

Although the original reproducer no longer exists, I was able to cook up
something similar.
The

Fix #49169: SoapServer calls wrong function, although "SOAP action" header is correct

Although the original reproducer no longer exists, I was able to cook up
something similar.
The problem is that there are two ways ext-soap currently looks up
functions:
1) By matching the exact function name; but this doesn't work if the
function name is not in the body.
2) By matching the parameter names.

Neither of these work when we don't have the function name in the body,
and when the parameter names are not unique. That's where we can use the
"SOAPAction" header to distinguish between different actions. This header
should be checked first and be matched against the "soapAction"
attribute in the WSDL. We keep the existing fallbacks such that the
chance of a BC break is minimized.
Note that since #49169 a potential target namespace is ignored right
now.

Closes GH-15970.

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