Keys are allowed to contain whitespace.

Fixes #45.

This is not a breaking change. All previous valid TOML documents
are still valid.

encode.go

@@ -10,7 +10,6 @@ import (
 	"strconv"
 	"strings"
 	"time"
-	"unicode"
 )
 
 type tomlEncodeError struct{ error }
@@ -512,10 +511,5 @@ func isValidKeyName(s string) bool {
 	if len(s) == 0 {
 		return false
 	}
-	for _, r := range s {
-		if unicode.IsSpace(r) {
-			return false
-		}
-	}
 	return true
 }

lex.go

@@ -2,6 +2,7 @@ package toml
 
 import (
 	"fmt"
+	"strings"
 	"unicode/utf8"
 )
 
@@ -112,6 +113,11 @@ func (lx *lexer) emit(typ itemType) {
 	lx.start = lx.pos
 }
 
+func (lx *lexer) emitTrim(typ itemType) {
+	lx.items <- item{typ, strings.TrimSpace(lx.current()), lx.line}
+	lx.start = lx.pos
+}
+
 func (lx *lexer) next() (r rune) {
 	if lx.pos >= len(lx.input) {
 		lx.width = 0
@@ -315,14 +321,14 @@ func lexKey(lx *lexer) stateFn {
 	// last non-whitespace character before the equals sign."
 	// Note here that whitespace is either a tab or a space.
 	// But we'll call it quits if we see a new line too.
-	if isWhitespace(r) || isNL(r) {
-		lx.emit(itemText)
+	if isNL(r) {
+		lx.emitTrim(itemText)
 		return lexKeyEnd
 	}
 
 	// Let's also call it quits if we see an equals sign.
 	if r == keySep {
-		lx.emit(itemText)
+		lx.emitTrim(itemText)
 		return lexKeyEnd
 	}
 
@@ -331,14 +337,10 @@ func lexKey(lx *lexer) stateFn {
 }
 
 // lexKeyEnd consumes the end of a key (up to the key separator).
-// Assumes that the first whitespace character after a key (or the '='
-// separator) has NOT been consumed.
+// Assumes that any whitespace after a key has been consumed.
 func lexKeyEnd(lx *lexer) stateFn {
 	r := lx.next()
-	switch {
-	case isWhitespace(r) || isNL(r):
-		return lexSkip(lx, lexKeyEnd)
-	case r == keySep:
+	if r == keySep {
 		return lexSkip(lx, lexValue)
 	}
 	return lx.errorf("Expected key separator %q, but got %q instead.",