fix fallback when symlink in AbstractAdapter#link_or_copy_file fails

[mraidel]

Paperclip::AbstractAdapter provides the link_or_copy_file method which first tries to do a symlink. When the symlinking raises an exception it is rescued and it tries again with copy. The problem is that the failed symlink leaves the tempfile instance in a bad state and you cannot read from it without reopening it.

Current behavior with a file which cannot be linked (for example because it is on another filesystem):

File.open("file_which_cannot_be_linked", "w") { |f| f.print("body") }

Paperclip.io_adapters.for(File.open("file_which_cannot_be_linked")).read
[paperclip] Trying to link file_which_cannot_be_linked to /tmp/d783e2fb96492f5a9f5e9632d2987a1120180131-3949-1vavjso                                                                                                
[paperclip] Link failed with Invalid cross-device link @ rb_file_s_link - (file_which_cannot_be_linked, /tmp/d783e2fb96492f5a9f5e9632d2987a1120180131-3949-1vavjso); copying link file_which_cannot_be_linked to /tm
p/d783e2fb96492f5a9f5e9632d2987a1120180131-3949-1vavjso    
Command :: file -b --mime 'file_which_cannot_be_linked'                              
=> ""

Behavior after fix:

Paperclip.io_adapters.for(File.open("file_which_cannot_be_linked")).read                                                                                                                  
[paperclip] Trying to link file_which_cannot_be_linked to /tmp/d783e2fb96492f5a9f5e9632d2987a1120180131-3939-1cp0zud                                                                                              
[paperclip] Link failed with Invalid cross-device link; copying link file_which_cannot_be_linked to /tmp/d783e2fb96492f5a9f5e9632d2987a1120180131-3939-1cp0zud                                         
Command :: file -b --mime 'file_which_cannot_be_linked'                                                                                                                                                             
=> "body"                                                                                                                                                                                        

[houndci-bot]

Line is too long. [86/80]

[houndci-bot]

Put a comma after the last parameter of a multiline method call.

[garettarrowood]

Thank you for diagnosing this issue and opening this solution! Very appreciated @mraidel !

[sidraval]

Thanks @mraidel ! See #2567