Types: fixed index types in modules.

Fixes the following type errors:
    Property ''xxx'' of type 'NjsByteString | undefined' is not
    assignable to string index type 'NjsStringLike'.ts(2411)

While string index signatures are a powerful way to describe the
"dictionary" pattern, they also enforce that all properties match
their return type.
diff --git a/ts/ngx_http_js_module.d.ts b/ts/ngx_http_js_module.d.ts
index d29b138..0440b7f 100644
--- a/ts/ngx_http_js_module.d.ts
+++ b/ts/ngx_http_js_module.d.ts
@@ -39,7 +39,7 @@
     readonly 'Warning'?: NjsByteString;
     readonly 'X-Forwarded-For'?: NjsByteString;
 
-    readonly [prop: string]: NjsByteString;
+    readonly [prop: string]: NjsByteString | undefined;
 }
 
 interface NginxHeadersOut {
@@ -76,7 +76,7 @@
 
     'Set-Cookie'?: NjsStringLike[];
 
-    [prop: string]: NjsStringLike | NjsStringLike[];
+    [prop: string]: NjsStringLike | NjsStringLike[] | undefined;
 }
 
 interface NginxVariables {
@@ -229,7 +229,7 @@
     readonly 'upstream_trailer_'?: NjsByteString;
     readonly 'uri'?: NjsByteString;
 
-    [prop: string]: NjsStringLike;
+    [prop: string]: NjsStringLike | undefined;
 }
 
 interface NginxSubrequestOptions {
diff --git a/ts/ngx_stream_js_module.d.ts b/ts/ngx_stream_js_module.d.ts
index 47799ba..67e24df 100644
--- a/ts/ngx_stream_js_module.d.ts
+++ b/ts/ngx_stream_js_module.d.ts
@@ -66,7 +66,7 @@
     readonly 'time_iso8601'?: NjsByteString;
     readonly 'time_local'?: NjsByteString;
 
-    [prop: string]: NjsByteString;
+    [prop: string]: NjsByteString | undefined;
 }
 
 interface NginxStreamCallbackFlags {