Unmarshal onto partial membershipContent

[S7evinK]

This fixes cases where we e.g. receive valid events, but are unable to unmarshal onto MembershipContent because of an invalid displayname (which we don't care about when authing events):

json: cannot unmarshal bool into Go struct field MemberContent.displayname of type string"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (85489e2) 59.13% compared to head (8e7c6de) 59.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #428      +/-   ##
==========================================
+ Coverage   59.13%   59.23%   +0.10%     
==========================================
  Files          51       51              
  Lines        7128     7129       +1     
==========================================
+ Hits         4215     4223       +8     
+ Misses       2526     2519       -7     
  Partials      387      387              

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