Replace serde_derive imports in test with imports from serde

[jonasbb]

bors merge

[codecov]

Codecov Report

Merging #437 (fbc9a96) into master (a8bce82) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #437   +/-   ##
=======================================
  Coverage   70.06%   70.06%           
=======================================
  Files          51       51           
  Lines        3404     3404           
=======================================
  Hits         2385     2385           
  Misses       1019     1019           

Impacted Files	Coverage Δ
serde_with/src/base64.rs	87.09% <ø> (ø)
serde_with/src/chrono.rs	66.01% <ø> (ø)
serde_with/src/hex.rs	76.92% <ø> (ø)
serde_with/src/json.rs	73.33% <ø> (ø)
serde_with/src/lib.rs	75.00% <ø> (ø)
serde_with/src/rust.rs	55.19% <ø> (ø)
serde_with/src/serde_conv.rs	100.00% <ø> (ø)
serde_with/src/with_prefix.rs	56.33% <ø> (ø)
serde_with/tests/with_prefix.rs	100.00% <ø> (ø)

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[bors]

Build succeeded:

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• Tests result