Issue title: Adding a currency sign in the price field for a download loses the value
 Issue body: If you enter "$5" in the price area for a download, on saving the value becomes $0. There's something in EDD filtering out that number. 

 Comments: 
 Comment 0: Report: https://easydigitaldownloads.com/support/topic/getting-some-issues/#post-244292

 Comment 1: Needs unit tests now.

 Comment 2: You mean update the existing ones to add a check for '$' in value?

 Comment 3: We need additional assertions that include currency signs (at least 5-10 difference currency signs).

 Comment 4: Hmmm hang on. I only added USD

 Comment 5: Yep, 5 - 10 please.

 Comment 6: Also, please test and confirm that it also sanitizes variable prices, both on save and on display.

 Comment 7: If it doesn't, wouldn't that fall under a different bug? The function itself didn't change, so such a bug would have been pre-existing and have more to do with variable pricing than the sanitization function... I believe...

 Comment 8: No, it's part of the same bug.

 Comment 9: @pippinsplugins ping... https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2319

{'source': 'GithubIssues', 'repo_name_with_owner': 'awesomemotive/easy-digital-downloads', 'issue_id': 34207654, 'repo_stars': 813.0}