sort options before converting to string

This makes the options string more predictable, and easier to compute by
other programs.  This is primarily  being done to enable signatures that
include option values (see #145).

CHANGELOG.md

@@ -10,6 +10,16 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### Changed
  - updated docker image to use go1.12 compiler and build imageproxy as a go module.
 
+ - options are now sorted when converting to string.  This is a breaking change
+   for anyone relying on the option order, and will additionally invalidate
+   most cached values, since the option string is part of the cache key.
+
+   Both the original remote image, as well as any transformations on that image
+   are cached, but only the transformed images will be impacted by this change.
+   This will result in imageproxy having to re-perform the transformations, but
+   should not result in re-fetching the remote image, unless it has already
+   otherwise expired.
+
 ### Removed
  - removed deprecated `whitelist` flag and `Proxy.Whitelist` struct field. Use
    `allowHosts` and `Proxy.AllowHosts` instead.

data.go

@@ -19,6 +19,7 @@ import (
 	"net/http"
 	"net/url"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 )
@@ -133,6 +134,7 @@ func (o Options) String() string {
 	if o.SmartCrop {
 		opts = append(opts, optSmartCrop)
 	}
+	sort.Strings(opts)
 	return strings.Join(opts, ",")
 }
 

data_test.go

@@ -33,19 +33,19 @@ func TestOptions_String(t *testing.T) {
 		},
 		{
 			Options{1, 2, true, 90, true, true, 80, "", false, "", 0, 0, 0, 0, false},
-			"1x2,fit,r90,fv,fh,q80",
+			"1x2,fh,fit,fv,q80,r90",
 		},
 		{
 			Options{0.15, 1.3, false, 45, false, false, 95, "c0ffee", false, "png", 0, 0, 0, 0, false},
-			"0.15x1.3,r45,q95,sc0ffee,png",
+			"0.15x1.3,png,q95,r45,sc0ffee",
 		},
 		{
 			Options{0.15, 1.3, false, 45, false, false, 95, "c0ffee", false, "", 100, 200, 0, 0, false},
-			"0.15x1.3,r45,q95,sc0ffee,cx100,cy200",
+			"0.15x1.3,cx100,cy200,q95,r45,sc0ffee",
 		},
 		{
 			Options{0.15, 1.3, false, 45, false, false, 95, "c0ffee", false, "png", 100, 200, 300, 400, false},
-			"0.15x1.3,r45,q95,sc0ffee,png,cx100,cy200,cw300,ch400",
+			"0.15x1.3,ch400,cw300,cx100,cy200,png,q95,r45,sc0ffee",
 		},
 	}