Apply assertions to VCRConfig's initial arguments

[aaronwolen]

This is mostly an academic change with zero functional impact for users but I think it's the "correct" approach for adding assertions to R6 classes, so it might be useful as a reference if nothing else.

In the current implementation assertions are only applied when updating the fields of an existing object, so it's possible to initialize an object with an invalid value:

config <- vcr:::VCRConfig$new(record = "once")
config$record <- 1
#> Error in check_record_mode(value): is.character(x) is not TRUE

vcr:::VCRConfig$new(record = 1)
#> <vcr configuration>
#>   Cassette Dir: .
#>   Record: 1
#>   URI Parser: crul::url_parse
#>   Match Requests on: method, uri
#>   Preserve Bytes?: FALSE
#>   Logging?: FALSE
#>   ignored hosts: 
#>   ignore localhost?: FALSE
#>   Write disk path:

With this new change, initial arguments are passed to the active binding rather than being assigned directly to their private field, so assertions are applied in both cases:

config <- vcr:::VCRConfig$new(record = "once")
config$record <- 1
#> Error in check_record_mode(value): is.character(x) is not TRUE

vcr:::VCRConfig$new(record = 1)
#> Error in check_record_mode(value): is.character(x) is not TRUE

^{Created on 2020-02-19 by the reprex package (v0.3.0)}

You have way more experience with R6 so I'm curious to see what you think and whether I'm missing something.

[codecov-io]

Codecov Report

Merging #156 into master will increase coverage by 2.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   75.51%   77.63%   +2.11%     
==========================================
  Files          36       36              
  Lines        1511     1511              
==========================================
+ Hits         1141     1173      +32     
+ Misses        370      338      -32

Impacted Files	Coverage Δ
R/configuration.R	64.91% <100%> (+28.07%)	⬆️

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[sckott]

good catch on making sure user inputs are checked on initialize as well.

using active binding on init seems smart to me.

ran tests and such and don't see any issues.