Handle files with spaces correctly in encoding_for

When passed a file name with a space, we end up calling
`file` without escaping the spaces (and other special characters).
This most likely causes calling `file` on a nonexistent file.
The upload then appears to hang, and eventually the job is terminated.

This commit fixes that.

lib/dpl/provider.rb

@@ -210,7 +210,7 @@ def detect_encoding?
     end
 
     def encoding_for(path)
-      file_cmd_output = `file #{path}`
+      file_cmd_output = `file '#{path}'`
       case file_cmd_output
       when /gzip compressed/
         'gzip'

spec/provider_spec.rb

@@ -150,7 +150,13 @@
   describe "#encoding_for" do
     example do
       path = 'foo.js'
-      expect(provider).to receive(:`).at_least(1).times.with("file #{path}").and_return('gzip compressed')
+      expect(provider).to receive(:`).at_least(1).times.with("file '#{path}'").and_return("#{path}: gzip compressed")
+      expect(provider.encoding_for(path)).to eq('gzip')
+    end
+
+    example do
+      path = 'file with a space'
+      expect(provider).to receive(:`).at_least(1).times.with("file '#{path}'").and_return("#{path}: empty")
       expect(provider.encoding_for(path)).to eq('gzip')
     end
   end