Replace (slower) block.call with (faster) yield

Performance optimization: `yield` with an implicit `block` is faster than `block.call`.
See http://youtu.be/fGFM_UrSp70?t=10m35s and the following benchmark:

```ruby
require 'benchmark/ips'

def fast
 yield
end

def slow(&block)
 block.call
end

Benchmark.ips do |x|
 x.report('fast') { fast{} }
 x.report('slow') { slow{} }
end

# => fast    154095 i/100ms
# => slow     71454 i/100ms
# =>
# => fast  7511067.8 (±5.0%) i/s -   37445085 in   4.999660s
# => slow  1227576.9 (±6.8%) i/s -    6145044 in   5.028356s
```
[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys
[ci skip] Add a note about headers overwriting

Since some headers can appear multiple times in an email it's required
to set it to nil first when you want to overwrite an existing one.

This commit add some information about this process.

Fix #15912
Merge branch 'master' of github.com:rails/rails