Serializer: Add 1 space, if necessary, around '=' in tpl transclusions

* So far, templates marked modified were serialized where template
  param values are not preceded by any whitespace -- the value was
  being emitted as is. This often results in visually not so pleasing
  serialization of multi-line params like "a    =foo".

  This patch changes the default to include a single space before
  and after the '=' char where necessary.

* This patch only changes the default for transclusions known to be
  template translucions (as opposed to parser functions, since we are
  yet to investigate if leading whitespace is semantically insignificant).

* Some examples of how this patch changes serialization in edit mode:

  {{echo|foo}}   ==> {{echo|foo}}
  {{echo|a=b}}   ==> {{echo|a = b}}
  {{echo|a = b}} ==> {{echo|a = b}}

  {{lc:FOO|a=b}} ==> {{lc:FOO|a=b}}
  {{{{echo|echo}}|a=b}} ==> {{{{echo|echo}}|a=b}}

Change-Id: I11cfed279d96a7e13f0c45601930e8d7dda9d717
Merge "Serializer: Add 1 space, if necessary, around '=' in tpl transclusions"