[C++] It is not necessary to add padding after the magic header in the FileWriter implementation

[asfimport]

See ARROW-998

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-1002. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #693

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 693
#693