avoiding the call on String#join

For the code given below:
```ruby
require 'rubygems'
require 'benchmark'

string_array = ['a', 'b', 'c', 'd']

def old_method(string_array)
  strings = []
  string_array.each { |part| strings << part.to_s }
  strings.join
end

def new_method(string_array)
  string = ''
  string_array.each { |part| string << part.to_s }
  string  
end



TIMES = 500_000
Benchmark.bmbm do |b|
  b.report('old') do
    TIMES.times do
      old_method(string_array)
    end
  end

  b.report('new') do
    TIMES.times do
      new_method(string_array)
    end
  end
end
```

Benchmark results are:

```
Rehearsal ---------------------------------------
old   0.710000   0.020000   0.730000 (  0.736706)
new   0.440000   0.000000   0.440000 (  0.438176)
------------------------------ total: 1.170000sec

          user     system      total        real
old   0.730000   0.020000   0.750000 (  0.755124)
new   0.430000   0.000000   0.430000 (  0.435200)
```