proposal: add control to change printing for failure output

[poy]

Problem

When working with certain types (e.g., []byte) the output is not helpful:

Expected call at some_test.go:25 doesn't match the argument at index 0.
        Got: [123 34 116 101 120 116 34 58 34 86 101 114 115 105 111 110 58 32 48 46 48 46 49 34 125]
        Want: [123 34 116 101 120 116 34 58 34 86 101 114 115 115 111 110 58 32 48 46 48 46 49 34 125]

This is made worse when using a custom matcher.

Expected call at /Users/poy/workspace/go/src/user/poy/chat-bots/registrar/registrar_test.go:25 doesn't match the argument at index 0.
        Got: [123 34 116 101 120 116 34 58 34 86 101 114 115 105 111 110 58 32 48 46 48 46 49 34 125]
        Want: is equal to map[text:0.0.1]

Proposal

Add a new interface GotFormatter that a custom matcher could implement. Before printing the failure's "Got" field, check to see if the matcher implements the new interface and use it to convert the value into a string.

type GotFormatter interface {
  Format(actual interface{}) string
}

[codyoss]

This feature has been added with #236