Added Json.dropNullValues.

[orium]

No description provided.

[orium]

Not sure if this is "too specific" to be added to Json, but it is something that I often use and can be useful for others (see #584 and #585).

Maybe it would be nicer to make it def dropNullValues(deep: Boolean = false): Json?

[codecov-io]

Codecov Report

Merging #983 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #983      +/-   ##
=========================================
- Coverage   85.28%   85.2%   -0.08%     
=========================================
  Files          75      75              
  Lines        2351    2352       +1     
  Branches      160     161       +1     
=========================================
- Hits         2005    2004       -1     
- Misses        346     348       +2

Impacted Files	Coverage Δ
...les/core/shared/src/main/scala/io/circe/Json.scala	78.34% <0%> (-0.51%)	⬇️
...rc/main/scala/io/circe/shapes/SizedInstances.scala	78.57% <0%> (-14.29%)	⬇️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	89.37% <0%> (-0.63%)	⬇️
.../core/shared/src/main/scala/io/circe/Decoder.scala	93.01% <0%> (+0.31%)	⬆️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	93.58% <0%> (+1.28%)	⬆️

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[poslegm]

It seems very useful for me! Any chance that it will be merged?

[travisbrown]

I've been wanting to avoid this and leave the presence or absence of null values as a printing-level issue, but there seems to be enough demand that I guess I should give that up. I'll merge this as-is, and if someone wants to add the deep version we can review it when it comes up.

[realvictorprm]

funny thing, this is just something I wanted to use and then I noticed that this was merged a day ago 😄 nice stuff 😃