Use xpath's string() to convert node-set to string

[duongnt]

To get username from SAML response, right now we execute an xpath expression, expect the results as a nodeset, and then return the first item in the nodeset. This behavior can be done completely in xpath using https://www.w3.org/TR/1999/REC-xpath-19991116/#function-string

This diff expects the xpath_username_location expression to be written in that way, so we could just return the whole result.

This fixes #46

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Signed-off-by: Thi Duong Nguyen (thiduongnguyen@gmail.com)

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 0.05%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   91.01%   91.06%   +0.05%     
==========================================
  Files           2        2              
  Lines         345      347       +2     
==========================================
+ Hits          314      316       +2     
  Misses         31       31

Impacted Files	Coverage Δ
samlauthenticator/samlauthenticator.py	91.04% <100%> (+0.05%)	:arrow_up:

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[distortedsignal]

Looks like this PR created a branch we haven't covered in a test case. Would you mind writing a test for it?

[duongnt]

@distortedsignal Could you please take another look at the PR?

[distortedsignal]

Sorry, it's been a really crazy day - one long lunch after an acquisition, people deleting branches in other repos - I'll get to this tonight. Promise.