Add tests for IndexMap and IndexSet

[jonasbb]

During the tests a missing implementation of DuplicateInsertsFirstWinsMap was noticed, but is now included.
indexmap was also missing the serde-1 feature flag.

bors merge

[bors]

Build failed:

• Tests result

[codecov]

Codecov Report

Merging #436 (9196836) into master (a8bce82) will increase coverage by 0.91%.
The diff coverage is 96.82%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   70.06%   70.98%   +0.91%     
==========================================
  Files          51       52       +1     
  Lines        3404     3467      +63     
==========================================
+ Hits         2385     2461      +76     
+ Misses       1019     1006      -13     

Impacted Files	Coverage Δ
serde_with/src/ser/impls.rs	93.75% <ø> (ø)
...e_with/src/duplicate_key_impls/first_value_wins.rs	57.14% <77.77%> (+7.14%)	⬆️
serde_with/src/de/impls.rs	72.88% <100.00%> (+0.23%)	⬆️
serde_with/tests/indexmap.rs	100.00% <100.00%> (ø)
...de_with/src/duplicate_key_impls/last_value_wins.rs	87.50% <0.00%> (+31.25%)	⬆️
...with/src/duplicate_key_impls/error_on_duplicate.rs	87.50% <0.00%> (+31.25%)	⬆️

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[jonasbb]

bors merge

[bors]

Build succeeded:

• Rustsec Audit
• Tests result