Refactor AttributeValueBase set_text method

[c00kiemon5ter]

This changeset is a refactor of the AttributeValueBase.set_text() method.
The rewrite is done in a declarative fashion. The mappings define the behaviour of the code, while the flow is common for all types.
The changeset also adds new tests that describe the behaviour of the function and make sure that the rewrite behaves the same way.

Related to #539 #543 #544 #546

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[c00kiemon5ter]

@cmurphy @jdavisturak @nebularazer feel free to report back the results if you test this patch.

[codecov-io]

Codecov Report

Merging #545 into master will increase coverage by 0.03%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
+ Coverage   65.48%   65.52%   +0.03%     
==========================================
  Files         103      103              
  Lines       25657    25598      -59     
==========================================
- Hits        16802    16772      -30     
+ Misses       8855     8826      -29

Impacted Files	Coverage Δ
src/saml2/saml.py	94.46% <92.85%> (+2.07%)	⬆️
src/saml2/time_util.py	86.82% <0%> (-0.6%)	⬇️
src/saml2/validate.py	76.07% <0%> (+2.35%)	⬆️

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[jonespm]

I tested based on the issue on #546. It's giving a different error now.

ValueError
Type and value do not match: xsd:string:<class 'str'>:urn:mace:dir:entitlement:common-lib-terms
/usr/local/lib/python3.5/site-packages/saml2/saml.py in _wrong_type_value, line 206
/usr/local/bin/python
3.5.5

I think it's still just checking types in the xs: namespace?

[jonespm]

Yes, @c00kiemon5ter the latest commit solves #546 and it works. Thanks!

[c00kiemon5ter]

@cmurphy @jdavisturak @nebularazer @jonespm just released v4.6.2 with this changeset.

[jdavisturak]

This fixes the issue I was seeing. Thank you!