Fix space issue in AntStructure.DTDPrinter

Task AntStructure renders a project DTD. In AntStructure class DTDPrinter, line 327, a single statement is used to concatenate a list of enumerated values which are valid for an attribute. The list is enclosed in parentheses. At line 348 the string "#IMPLIED" is appended to all attribute definitions regardless of type. For other types like Boolean and CDATA, a space is appended. Not for these enumerations. This results in a DTD validation error. Ref [XERCES definition](https://xerces.apache.org/xerces-j/apiDocs/org/apache/xerces/utils/XMLMessages.html#MSG_SPACE_REQUIRED_BEFORE_DEFAULTDECL_IN_ATTDEF)

The full message displayed in VSCode may be different from elsewhere:

    White space is required before the attribute default in the declaration
    of attribute "x" for element "y".
    xml(MSG_SPACE_REQUIRED_BEFORE_DEFAULTDECL_IN_ATTDEF)

This simple one-line change appends the space character required to avoid that error.

**Note:** This should not be modified into a change on the `joinAlts` Collector which surrounds values in parentheses. That Collector is used elsewhere where regex syntax is applied immediately after. See line 298. That asterisk cannot be separated from the values with a space. The space must only be applied to line 327.

CONTRIBUTORS

@@ -433,6 +433,7 @@ Tom Eugelink
 Tom May
 Tomasz Bech
 Tomáš Zezula
+Tony Gravagno
 Trejkaz Xaoza
 Ulrich Schmidt
 Uwe Schindler

WHATSNEW

@@ -34,6 +34,9 @@ Fixed bugs:
    then the forked mode launch would fail.
    Bugzilla Report 63958
 
+ * Fixes an issue in AntStructure where an incorrect DTD was being generated.
+   Github Pull Request #116
+
 Other changes:
 --------------
 

contributors.xml

@@ -1783,6 +1783,10 @@
     <first>Tomáš</first>
     <last>Zezula</last>
   </name>
+  <name>
+    <first>Tony</first>
+    <last>Gravagno</last>
+  </name>
   <name>
     <first>Trejkaz</first>
     <last>Xaoza</last>

src/main/org/apache/tools/ant/taskdefs/AntStructure.java

@@ -324,7 +324,7 @@ public class AntStructure extends Task {
                             || !areNmtokens(values)) {
                             sb.append("CDATA ");
                         } else {
-                            sb.append(Stream.of(values).collect(joinAlts));
+                            sb.append(Stream.of(values).collect(joinAlts)).append(" ");
                         }
                     } catch (final InstantiationException | IllegalAccessException ie) {
                         sb.append("CDATA ");
@@ -337,7 +337,7 @@ public class AntStructure extends Task {
                             sb.append("CDATA ");
                         } else {
                             sb.append(Stream.of(values).map(Enum::name)
-                                .collect(joinAlts));
+                                .collect(joinAlts)).append(" ");
                         }
                     } catch (final Exception x) {
                         sb.append("CDATA ");