file:// => file:///
WickedPdfTempfile.new use temp_path argument

You can use pdf_from_string with an option :temp_path. If temp_path available WickedPdfTempfile use this path. 
Merge pull request #176 from sharpyfox/fix_travis

fix for travis-ci integration
Resolve merge conflict from Pull #175
Update lib/pdf_helper.rb

conditional assignment of :file option was not correctly implemented. 
Still ended up with :file => nil in the hash.
Merge pull request #177 from linoj/master

Missed conditional assignment of :file option
Add WickedPdf::VERSION constant and update version to 0.9.3
Fix issue with WickedPdfTempfile path arg on Ruby 1.8

Ruby 1.8's Tempfile doesn't tolerate nil as the tempfile path,
however, both old and newer versions of Tempfile will
accept an array as the first arg for a prefix and suffix.
Update version to 0.9.4
Add ability to set options in Middleware
Fix issue parsing middleware options
Add support for :disposition => 'attachment' in middleware
Check if logger isn't nil
Merge pull request #183 from zewelor/master

Check if logger isn't nil
Add command stderr to exception message
Update README.md

add format
Merge pull request #198 from lidaobing/patch-1

Add syntax highlighting to README
Fix for precompiled assets when using Proc as asset_host
Converting tabs into spaces
Merge pull request #201 from gogogarrett/patch-1

Converting tabs into spaces