[SPARK-16212][STREAMING][KAFKA] code cleanup from review feedback

[koeninger]

What changes were proposed in this pull request?

code cleanup in kafka-0-8 to match suggested changes for kafka-0-10 branch

How was this patch tested?

unit tests

[SparkQA]

Test build #61248 has finished for PR 13908 at commit 576a0e4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61249 has finished for PR 13908 at commit 647c2af.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61251 has finished for PR 13908 at commit d8a1ba0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61508 has finished for PR 13908 at commit ea35641.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[tdas]

LGTM. Unless you plan to make more changes.

[koeninger]

I don't plan to make any more changes... I kind of doubt feedback on your #13996 is going to result in any changes that would also apply to 0.8

[tdas]

Cool. Merging this to master