Replace AS::TimeWithZone#since with alias to +

[claudiob]

Stems from Google group discussion.

Currently AS::TimeWithZone has two methods to add an interval to a time: +(other) and since(other) (docs).

The two methods are "pretty much" equivalent in every case:

1. When adding any interval to an AS::TimeWithZone representing a Time:

   t = Time.now.in_time_zone       #=> Thu, 04 Dec 2014 18:56:28 EST -05:00
   t + 1 == t.since(1)             #=> true
   t + 1.day == t.since(1.day)     #=> true
   t + 1.month == t.since(1.month) #=> true

2. When adding any interval to an AS::TimeWithZone representing a Date:

   d = Date.today.in_time_zone     #=> Thu, 04 Dec 2014 00:00:00 EST -05:00
   d + 1 == d.since(1)             #=> true
   d + 1.day == d.since(1.day)     #=> true
   d + 1.month == d.since(1.month) #=> true

3. When adding any interval to an AS::TimeWithZone representing a DateTime:

   dt = DateTime.now.in_time_zone    #=> Thu, 04 Dec 2014 18:57:28 EST -05:00
   dt + 1 == dt.since(1)             #=> true
   dt + 1.day == dt.since(1.day)     #=> true
   dt + 1.month == dt.since(1.month) #=> false

As you can see, the only case in which they differ is when the interval added to a DateTime is in a format like 1.month.

However, this usage of "since" is explicitly discouraged by the documentation of DateTime#since:

Returns a new DateTime representing the time a number of seconds since the instance time.
Do not use this method in combination with x.months, use months_since instead!

And indeed, following this recommendation the correct result is returned:

dt + 1.month == dt.months_since 1 #=> true

Therefore, my proposal is to remove the method definition of TimeWithZone#since and instead replace it with a simple alias_method :since, :+.

The rationale is that the only case where they differ is a case that is explicitly discouraged as "wrong".

In my opinion, having two methods named since and + and having to figure out exactly what the difference is makes the codebase more confusing.

However, I understand this PR is "subjective", so if you feel like it's better to ignore this, feel free to close the PR.

Thanks!