Change input-decoder interface to return YAML request body

[jronak]

Prior to this change input decoder exposed method which returns JSON body, but existing Serializer interface already exposes methods with YAML body. This change upholds the consistency between all the interfaces and also avoids the lossy conversion from YAML to JSON type.

Fixes https://github.com/yarpc/yab/pull/314#discussion_r565766802

[codecov[bot]]

Codecov Report

Merging #321 (99bd1d3) into dev (2a495ad) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #321      +/-   ##
==========================================
+ Coverage   90.27%   90.33%   +0.06%     
==========================================
  Files          52       52              
  Lines        2590     2587       -3     
==========================================
- Hits         2338     2337       -1     
+ Misses        178      177       -1     
+ Partials       74       73       -1     

Impacted Files	Coverage Δ
encoding/inputdecoder/decoder.go	100.00% <100.00%> (+8.69%)	:arrow_up:
encoding/protobuf.go	92.72% <100.00%> (ø)

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[prashantv]

Does this support multiple YAML documents? E.g.,

---
bar: 1
---
bar: 2

https://www.ribbybibby.me/posts/multiple-yaml-documents/

[jronak]

Does this support multiple YAML documents? E.g.,

---
bar: 1
---
bar: 2

https://www.ribbybibby.me/posts/multiple-yaml-documents/

Yup, --- is used as a delimiter by yaml decoder and it can parse multiple yaml docs from the same input reader.

Linking to a gRPC stream integration test which uses multiple YAML documents - https://github.com/yarpc/yab/blob/dace80fcc1439a7a161713a4f7b801ec738d702d/integration_test.go#L493-L500