`as.data.frame()` methods don't match the generics

[capnrefsmmat]

R CMD check reports:

W  checking S3 generic/method consistency (660ms)
   as.data.frame:
     function(x, row.names, optional, ...)
   as.data.frame.covidcast_data_signal_list:
     function(signals, ...)
   
   as.data.frame:
     function(x, row.names, optional, ...)
   as.data.frame.covidcast_data_source_list:
     function(sources, ...)
   See section ‘Generic functions and methods’ in the ‘Writing R
   Extensions’ manual.

These methods need to accept exactly the same argument names as the base as.data.frame(), plus optionally any additional ones. I've fixed a couple other generics in my branch, but these are trickier because we need to decide what to do with row.names and optional.

[dshemetov]

Related #112