Generalize handling of arguments with binary value types

[aoberoi]

Summary

based on #513 - as this PR states, this should allow methods like users.setPhoto to also work properly.

fixes #452

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #519 into master will decrease coverage by 0.28%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
- Coverage    79.7%   79.42%   -0.29%     
==========================================
  Files           6        6              
  Lines         271      277       +6     
  Branches       42       42              
==========================================
+ Hits          216      220       +4     
- Misses         38       39       +1     
- Partials       17       18       +1

Impacted Files	Coverage Δ
src/WebClient.ts	81.25% <88.88%> (-0.64%)	⬇️

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[aoberoi]

an open point we should resolve before merging this.

[aoberoi]

blocker: i just tried using this implementation with a Buffer (with or without a filename argument) and it failed with the following error:

   { ok: false,
     error: 'no_file_data',
     acceptedScopes: [ 'files:write:user', 'post' ] } }

[aoberoi]

okay, i've learned an interesting thing. the following is true for files.upload (i haven't checked against other methods, but i will):

if in the multipart body, the part that contains the binary data does not specify a filename attribute in the header, the upload will fail. if another part contains a filename that will override whatever was specified in the filename attribute of the first part.

[aoberoi]

and the same is true for users.setPhoto. that is that if no filename attribute is specified, it will fail. in this case, there's no need for an additional filename part, since the filename is generated by Slack.

[aoberoi]

side note: love it when you implement a new feature with more deletions than additions 💯

[Roach]

Should we add a note to the docs for this?

[aoberoi]

heh, if we had generated docs for the individual methods at all, then i'd say yes.