Fix comparison bug when comparing non-numeric prereleases

Reference: hashicorp#61
This fixes a bug when comparing non-numeric prereleases of different set
sizes. Makes it so that X.Y-alpha is considered smaller than X.Y-alpha.beta
because a larger set of pre-release fields should have a higher
precedence than a smaller set if all of the preceding identifiers are
equal.

When considering prerelease parts that are empty, use the other to
decide regardless if it is numeric or not. If our self part is empty
then the other part must be the larger set. If the other part is empty
then the self part must be the larger set.

Also fix and add a few additional test cases.

version.go

@@ -212,17 +212,11 @@ func comparePart(preSelf string, preOther string) int {
 
 	// if a part is empty, we use the other to decide
 	if preSelf == "" {
-		if otherNumeric {
-			return -1
-		}
-		return 1
+		return -1
 	}
 
 	if preOther == "" {
-		if selfNumeric {
-			return 1
-		}
-		return -1
+		return 1
 	}
 
 	if selfNumeric && !otherNumeric {

version_test.go

@@ -190,7 +190,9 @@ func TestComparePreReleases(t *testing.T) {
 		{"3.0-alpha.3", "3.0-rc.1", -1},
 		{"3.0-alpha3", "3.0-rc1", -1},
 		{"3.0-alpha.1", "3.0-alpha.beta", -1},
-		{"5.4-alpha", "5.4-alpha.beta", 1},
+		{"5.4-alpha", "5.4-alpha.beta", -1},
+		{"5.4-beta", "5.4-alpha.beta", 1},
+		{"5.4-beta.2", "5.4-alpha.beta", 1},
 		{"v1.2-beta.2", "v1.2-beta.2", 0},
 		{"v1.2-beta.1", "v1.2-beta.2", -1},
 		{"v3.2-alpha.1", "v3.2-alpha", 1},