Use io.ReadFull to ensure that generated jwk match the expected keysize

[vihangm]

io.Reader.Read docs say the following:

If some data is available but not len(p) bytes, Read conventionally returns what is available instead of waiting for more.

We need to use io.ReadFull or rand.Read (which in turn calls ReadFull) to ensure that we fill the entire buffer.

[codecov]

Codecov Report

Attention: 29 lines in your changes are missing coverage. Please review.

Comparison is base (0a15b4d) 72.00% compared to head (3a96973) 72.40%.
Report is 34 commits behind head on develop/v2.

Files	Patch %	Lines
internal/keyconv/keyconv.go	42.42%	16 Missing and 3 partials ⚠️
jwt/openid/birthdate.go	76.47%	4 Missing ⚠️
jwe/jwe.go	91.66%	2 Missing and 1 partial ⚠️
jwe/internal/aescbc/aescbc.go	85.71%	1 Missing and 1 partial ⚠️
jwe/options_gen.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           develop/v2    #1064      +/-   ##
==============================================
+ Coverage       72.00%   72.40%   +0.39%     
==============================================
  Files              93       93              
  Lines           13795    13896     +101     
==============================================
+ Hits             9933    10061     +128     
+ Misses           3044     3020      -24     
+ Partials          818      815       -3     

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[lestrrat]

LGTM. Thanks!