Optimize TimeWithZoneTest#strftime

Replace TimeWithZoneTest#strftime, with a version that doesn't create unnecessary String instances and avoid unnecessary method calls. 


Some benchmarks:
```ruby
#Worse scenario, every formating option present (from TimeWithZone) in use: 73% faster
Benchmark.ips do |bm|
  bm.report('original') { @twz.strftime_original('%Y-%m-%d %H:%M:%S %Z %z %:z %::z') }
  bm.report('optimized') { @twz.strftime('%Y-%m-%d %H:%M:%S %Z %z %:z %::z') }
end

#Calculating -------------------------------------
#            original      2331 i/100ms
#           optimized      3990 i/100ms
#-------------------------------------------------
#            original    24536.5 (±1.1%) i/s -     123543 in   5.035705s
#           optimized    42606.0 (±1.2%) i/s -     215460 in   5.057748s

#With 2 formating options: 2.9 times faster
Benchmark.ips do |bm|
  bm.report('original') { @twz.strftime_original('%Y-%m-%d %H:%M:%S %Z %z') }
  bm.report('optimized') { @twz.strftime('%Y-%m-%d %H:%M:%S %Z %z') }
end

#Calculating -------------------------------------
#            original      2598 i/100ms
#           optimized      6959 i/100ms
#-------------------------------------------------
#            original    27054.3 (±1.0%) i/s -     137694 in   5.090060s
#           optimized    79036.4 (±1.4%) i/s -     396663 in   5.019817s


#With 1 formating options: 5.5 times faster
Benchmark.ips do |bm|
  bm.report('original') { @twz.strftime_original('%Y-%m-%d %H:%M:%S %Z') }
  bm.report('optimized') { @twz.strftime('%Y-%m-%d %H:%M:%S %Z') }
end

#Calculating -------------------------------------
#            original      2742 i/100ms
#           optimized     12840 i/100ms
#-------------------------------------------------
#            original    29109.9 (±1.4%) i/s -     148068 in   5.087531s
#           optimized   160027.8 (±1.1%) i/s -     808920 in   5.055439s



#With no formating options: 8.1 times faster
Benchmark.ips do |bm|
  bm.report('original') { @twz.strftime_original('%Y-%m-%d %H:%M:%S') }
  bm.report('optimized') { @twz.strftime('%Y-%m-%d %H:%M:%S') }
end

#Calculating -------------------------------------
#            original      2974 i/100ms
#           optimized     19257 i/100ms
#-------------------------------------------------
#            original    31817.4 (±1.0%) i/s -     160596 in   5.047983s
#           optimized   258589.2 (±3.2%) i/s -    1309476 in   5.070831s
```