[R] Feather api fixes

[asfimport]

e.g. feather_table_writer should be FeatherTableWriter, ...

Reporter: Romain Francois / @romainfrancois
Assignee: Romain Francois / @romainfrancois

PRs and other links:

• GitHub Pull Request #3106

_{Note: This issue was originally created as ARROW-3942. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3106
#3106