Super minor: Add require for mergeCombiners in combineByKey

We changed the behavior in 0.9.0 from requiring that mergeCombiners be null when mapSideCombine was false to requiring that mergeCombiners *never* be null, for external sorting. This patch adds a require() to make this behavior change explicitly messaged rather than resulting in a NPE.

Author: Aaron Davidson <aaron@databricks.com>

Closes #623 from aarondav/master and squashes the following commits:

520b80c [Aaron Davidson] Super minor: Add require for mergeCombiners in combineByKey
(cherry picked from commit 3fede4831eeb7d36d4f8fa4aaa02ad0cc8b4b09e)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
SPARK-1111: URL Validation Throws Error for HDFS URL's

Fixes an error where HDFS URL's cause an exception. Should be merged into master and 0.9.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #625 from pwendell/url-validation and squashes the following commits:

d14bfe3 [Patrick Wendell] SPARK-1111: URL Validation Throws Error for HDFS URL's

(cherry picked from commit 45b15e27a84527abeaa8588b0eb1ade7e831e6ef)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
Merge branch 'branch-0.9' of https://github.com/apache/incubator-spark into csd-2.10