fix asindices function used in sortheader to support duplicate values…

[arturponinski]

… passed to spec

This PR has the objective of supporting duplicate column names in sortheader Closes: https://github.com/petl-developers/petl/issues/392

Changes

1. Change asindices to support passing of duplicate values when sorting non-unique column headers

Checklist

Use this checklist for assuring the quality of pull requests that include new code and or make changes to existing code.

• [ ] Source Code rules apply:
  • [x] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [x] Using atomic commits when possible
  • [ ] Commits are reversible when possible
  • [x] There is no incomplete changes in the pull request
  • [ ] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [x] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [x] Ready to review
  • [ ] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 1813637635

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 91.975%

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Totals
Change from base Build 1813620437:	0.1%
Covered Lines:	12481
Relevant Lines:	13570

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💛 - Coveralls