目标存储路径是文件，而不是文件夹情况下的异常处理设计不太合理

[AzureFatty]

在DownloadTask.java中

try {
    ...
    if (file.exists() && file.isFile()) {
    // it have already provided file for it.
	throw new IllegalArgumentException("If you want filename from "
					+ "response please make sure you provide path is directory " + file.getPath());
    }
    ...
} finally {
    this.id = OkDownload.with().breakpointStore().findOrCreateId(this);
}

此处判断file.exists() && file.isFile()抛出IllegalArgumentException异常，
当此判断条件成立时，因为加了finally，所以代码还会执行
this.id = OkDownload.with().breakpointStore().findOrCreateId(this);
而此时会抛出

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.xxx.sample.webdownloader, PID: 16251
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.liulishuo.okdownload.core.download.DownloadStrategy$FilenameHolder.get()' on a null object reference
        at com.liulishuo.okdownload.DownloadTask.getFilename(DownloadTask.java:230)
        at com.liulishuo.okdownload.core.breakpoint.KeyToIdMap.generateKey(KeyToIdMap.java:63)
        at com.liulishuo.okdownload.core.breakpoint.KeyToIdMap.get(KeyToIdMap.java:43)
        at com.liulishuo.okdownload.core.breakpoint.BreakpointStoreOnCache.findOrCreateId(BreakpointStoreOnCache.java:146)
        at com.liulishuo.okdownload.core.breakpoint.BreakpointStoreOnSQLite.findOrCreateId(BreakpointStoreOnSQLite.java:100)
        at com.liulishuo.okdownload.DownloadTask.<init>(DownloadTask.java:190)
        at com.liulishuo.okdownload.DownloadTask$Builder.build(DownloadTask.java:838)

从而掩盖了
throw new IllegalArgumentException("If you want filename from " + "response please make sure you provide path is directory " + file.getPath());
的异常信息。

导致追查问题时不方便。

[Jacksgong]

感谢反馈。

[Jacksgong]

该问题已经在1.0.2-SNAPSHOT上修复。